Final answer:
The I band gets smaller when a sarcomere contracts, while the A band stays the same width.
Step-by-step explanation:
When a sarcomere contracts, the portion of the band that gets smaller is the I band. The I band consists of only thin filaments and shortens during muscle contraction. This occurs as the Z lines move closer together, and as a result, the distance between consecutive Z discs decreases. Conversely, the A band remains the same width during the contraction process. At full contraction, the thin filaments overlap more with the thick filaments, increasing the zone of overlap and pulling the actin towards the center of the sarcomere.